MLR-R and MLR-S gene products are expressed on different lymphoid cells.

The role of human B and T lymphocytes in the mixed lymphocyte reaction (MLR) was investigated using T- and B-enriched populations obtained either by centrifugation of E rosetting cells over a Ficoll-Isopaque gradient or after B or T cells had been killed by specific antihuman T lymphocyte antigen or antihuman B lymphocyte and monocyte antigen sera, respectively, in the presence of complement. Purified T lymphocytes responded to allogenic cells whereas they were unable to stimulate the MLR; in contrast, purified B lymphocytes were not activated but were found to be good stimulators. Treatment of the MLR, at the time of addition of tritiated thymidine, by antihuman T lymphocyte antigen serum and complement completely suppressed the thymidine incorporation, indicating that the bulk of the proliferative response was supported by the T lymphocytes. Additional experiments with populations depleted of complement or Fc receptor-bearing lymphocytes did not suggest that these B lymphocyte subpopulations played any major role in the MLR.